1. Field of the invention
The present invention relates to a printed circuit board capable of preventing electromagnetic interference. More specifically, the present invention relates to a printed circuit board capable of preventing electromagnetic interference comprising a first electrically conductive layer which forms a signal pattern and a ground pattern, an insulation layer covering the first conductive layer, and a second electrically conductive layer formed on the insulation layer to be connected to the ground pattern.
2. Description of the prior art
Digital IC's such as a microcomputer, microprocessor, etc., are used in various electronic equipment as well as a personal computers, video game machines, etc. Where such a digital IC is mounted on a printed circuit board, the printed circuit board itself becomes a radiation source, and therefore, noise in the form of electromagnetic interference (EMI) is radiated in the air (a) from the printed circuit board itself or (b) through an antenna which is formed by a cord or cable connected to the printed circuit board. If no attempt is made to reduce the EMI noise, the EMI noise functions as an interference radio wave to other electronic components, that is, the EMI noise may cause reception noise in a television receiver or various control components to malfunctions. Therefore, the EMI noise must be reduced as much as possible.
Conventionally, with respect to the above described cause (a), a shielding technique is utilized. For example, a printed circuit board is covered by a shield plate such as a metal plate, expanded metal or the like which is connected to the ground so as to pass the EMI noise to the ground therethrough, whereby the EMI noise can be prevented from being radiated outside. In addition, with respect to the above described cause (b), a connector which incorporates a specific capacitor is utilized for connecting the cord or cable. However, in such prior art, it is not possible to completely prevent the EMI noise.
In addition, for example, in Japanese Utility Model Publication No. 55-29276, one example of a method for shielding is disclosed, wherein a silver paste is formed on the base plate to shield the base plate. However, this method is merely a modification of the above described shielding technique and directed to reduce spurious noise rather than the EMI noise. Such spurious noise has low-frequency components less than 10 MHz and is regularly generated. Therefore, the method disclosed in Japanese Utility Model Publication No. 55-29276 is effective for the spurious noise but is not effective for the EMI noise from a digital IC, which has very high-frequency components of 30-1000 MHz and is generated irregularly. Furthermore, the method disclosed in Japanese Utility Model Publication No. 55-29276 can not deal with the connection of the cord or cable.
Therefore, a novel printed circuit board was proposed capable of preventing EMI in accordance with a novel idea in Japanese Patent Laid-open No. 62-213192 corresponding to the inventors' assignee's U.S. Pat. No. 4,801,489. Thus printed circuit board capable of preventing the EMI includes an insulation layer formed on an insulating base plate so as to cover a first electric conductive layer formed on the insulating base plate, and a second electric conductive layer is further formed on the insulation layer by means of printing of a copper ink or paste. The proposed printed circuit board is effective in some cases, but it is desired to further improve the EMI noise preventing function of a printed circuit board.